A composite tension member and method for manufacturing same, and more particularly a composite tension member formed of a plurality of parallel composite fiber rods bundled together in an intermediate composite cable portion with the rods either splayed out into termination fittings by a cone and attached thereto with adhesives and by friction forces, or imbedded in a wedge or frustum plug, which wedge or plug is fitted into a termination fitting.
There are a variety of applications which require strong yet lightweight tension members, such as standing rigging members for sailboats, industrial structure bracing, lifting cables, mooring cables for offshore drilling platforms, and bridge tendons, to name a few.
Particularly in performance-sensitive areas such as competitive sailboat racing, the ability to provide high tensile strength, lightweight and small-profile tension members is particularly attractive since reducing the weight above the deck of a vessel and reducing the profile of the rigging will reduce windage, and therefore give the sailboat a competitive advantage. Presently, in the yachting world, rigging material such as stainless steel cable and stainless steel rod is widely used for holding the mast upward keeping the mast to the desired straightness or degree of bend. When stainless steel cable sailboat rigging is constructed, the individual strands of the cable can be attached to terminating fittings on the end of the cables, e.g., by mechanical clamping or swaging. Although stainless steel cable and stainless steel rod are relatively strong, they suffer from various drawbacks. First, stainless steel, while strong, is still relatively heavy. Also, stainless steel cable and rod are subject to stretching (either elastic or permanent deformation). Furthermore, there are often situations where the stainless steel cable or rod do not extend along completely straight runs, and must bend at certain points. These bends create localized areas of weakness. For example, in sailboat applications, certain stainless steel cables and rods pass are used as continuous upper shrouds, where they pass around mast spreaders, and are angled in these areas. In these cases, the cables and rods are weakened where they pass around the end of the spreaders, thereby compromising the strength of the continuous upper shrouds.
Other problems with stainless steel rigging are corrosion and work hardening. Until about twenty years ago, stainless steel wire rope for standing rigging was the norm. However, wire rope experiences “permanent stretch” which is caused by settling of the wires in the rope or strand as an initial load is applied. While some permanent stretch is removed during the wire rope manufacturing process, a tension member made of wire rope almost always needs to be re-tensioned after some use. Because of these problems, there was a shift to using stainless steel rod for sailboats and other performance-driven applications. The advantages of stainless steel rod are less stretch and higher strength than stainless steel wire rope. This is because rod has more cross sectional material for a given diameter and there is not the stretch effect of a twisted wire rope. However, when the shift was made to stainless steel rod rigging there were numerous failures. There were two primary reasons for rod rigging failures. First, dew and salt water would get down into the termination fitting and cause stress corrosion cracks. Second, there were work hardening fatigue failures. The fatigue failures resulted from the rod rigging not being flexible like wire rope at the terminations. Consequently, the rod was continuously bent back and forth where the termination at the mast became very rigid. Others have attempted to address this problem by providing flexible joints for the end of the rod rigging with some success. In contrast to stainless steel, carbon composite has a much improved fatigue life, and will not experience stress corrosion cracking.
Another problem with metals is that they begin to yield at a lower level before they ultimately fail. In contrast, carbon composites keep accepting load and perform as designed without yielding almost right up to their failure point.
Directional composite materials such as carbon fiber, glass fiber, Kevlar® fiber, Aramid fiber, or other fibers, combined with a polymer resin matrix, offer very high tensile strength with less weight than conventional metallic materials. The means to manufacture monolithic rod members from composite members of suitable size to handle the tensile load required, for example, of sailboat masts, has been in existence for some time. However, it is difficult to attach termination fittings to a large monolithic composite rod. Conventional methods of attaching termination fittings to steel riggings, such as swaging, will not work for composite materials because the swaging operation will crush the composite monolithic rod. Adhesive bonding for connecting monolithic composite rods to termination fittings at each end for a rigging member does not have sufficient tensile strength except for small rod sizes (e.g., 3 millimeters (⅛″) diameter or less) where the composite rod tensile strength does not exceed the strength of available adhesives. Moreover, in situations where the tension member will be locally bent, weaknesses are created. There accordingly remains a need for a solution to these problems.